Cathode ray tube



July 23, 1940.

P. GGRLICH ET AL 2, ,1

- CATHODE RAY TUBE Filed Dec. 2, 1937 J i i i i i I *"l'll'l'fill" Ju e/220m Peal, (i'rZL'ch Ward/z Pia/{e Patented July 23, 1940 UNETED STATES ear-EH orrice many, assignors to Zeiss Y Ikon Aktiengesellschaft, Dresden, Germany Application December 2, 1937, -Serial No. 177,682

- In Germany December 18, 1936 2 Claims.

The invention relates to improvements in cathode ray tubes, in particular Braun tubes.

It is anobject of the invention to provide a method and "means for reducing the chromatic 5 aberrations of electron lenses used in Braun tubes. 7

The electrons emitted from a source of electrons possess a characteristic variety of velocities depending upon the manner of their generation.

10 This unequality of speeds for instance forelectrons produced by a glow discharge is different from that produced by an irradiance of the body with light. Corresponding to this variety of speeds the electrons leaving the source at different angles with respect to the optical axis possess different radial speed components. It is I now well known, that the focal length of all conventional electric or magnetic condenser lenses is increased when the initial speed of the electronsis increased. This means, however, that at a greater distance'ffrom their source those electrons having a high radial speed component are spaced from the optical axis a greater distance than other electrons which leave the source at a comparatively smaller speed. Owing to the above mentioned property of the conventional electric or magnetic lenses, the common point of intersection of all of the paths of those electrons which have a high speed of emission and which 3 owing to their high radial speed component, are emitted from the source at a great angle will be positioned a greater distance from the source than the common point of intersection of all of those paths of the electrons which have a lower initial speed. It is, however, desirable that the paths of all electrons should intersect each other in one point only.

In accordance with the invention this object is obtained by interposing in the path of the electrons a lens having a substantial spheric aberration. Such a lens has the property of reiracting the rays which are farther away from the optical axis to a greater degree than those rays which are closer to the optical axis. It is therefore possible to bring the intersecting point of the paths of the electrons having a high 'initial speed almost to coincidence with the intersecting point of the paths of the electrons having a lower initial speed and to reduce thereby 50 the chromatic error, accomplishingone of the principal objects of the invention.

According to investigations the spheric error will become substantial when the diameter of the electron beam in the plane of the reproducing 55 lens is approximately one half of the diameter (Clj 250 -215) of aperture of said lens or even larger. It is accordingly another objectof the invention to provide the lens systems of cathode ray tubes with apertures of "relatively small diameter as compared withthe diameter of the electron beam. 5 Another object of the invention is to provide a method and means for adjusting or controlling the degree to which the chromatic error may be reduced. This is accomplished by varying the 'spheric error of the reproducing lens. -10

Another I object of the invention is to adjust the spheric error to the desired degree, without changing the'focal length of the entire system, by providing a plurality of coaxially mounted annular electrodes each of which hasapplied there- 15 toa tension of such value as is necessary to ob tain the desired spheric error.

iStill anotherobject of the invention is to pro- 'vide for the'above mentioned purpose a lens system, in which .the electron beam passes first 20 through a lens with small spheric error and then through a second lens having an adjustable spheric error.

With these and other objects in view as may appear hereinafter, the invention resides in cer- 5 tain improvements, the novel features of which will be more fully described with reference to the accompanying drawing, in which Fig. 1 illustrates diagrammatically a Braun tube provided with a spheric non-corrected elec- 30 tron lens for reducing chromatic aberration.

Fig. 2 illustrates diagrammatically a modified arrangement of the spheric non-corrected lens adapted for a Braun tube similar to the one shown in Fig. 1, and

Fig. 3 illustrates diagrammatically another modified arrangement of the spheric non-corrected lens.

Referring to Fig. 1 the vessel l of the Braun tube contains an equipment material for the present invention a cathode 2, a control electrode 3, an electric lens 4, the diaphragm 5 and 6, and electric lens I and the anode 8. The cathode 2 is heated by a battery 9. A battery Ill supplies the different voltages required for the electrodes and is for this purpose shunted by a potentiometer II from which the various taps lead to the respective electrodes. It will be noted that the diameter D1 of the electron beam, where it passes through the lens I, is larger than onehalf of the diameter D2 of the aperture of lens 1. Other equipment which may be present in the tube is omitted from the drawing to simplify the illustration.

According to the modification illustrated in 5g Fig. 2, the electron lens is composed of a number of axially alined annular electrodes 12. For the sake of clearness this assembly of electrodes only is illustrated as being most important for the present invention. The electron beam may be produced in a manner similar of that of Fig. 1 and passes as indicated by the full lines with the arrows thereon through the diaphragm 5 and into the space immediately behind the same. Then the electron beam passes through the series of axially alined annular electrodes l2, each electrode having applied thereto a different potential taken from the potentiometer l3. The entire series of electrodes I2 constitutes a spheric noncorrected electron lens whose spheric aberrations may be adjusted by varying the potentials applied to the electrodes without. changing the focal length of the entire system.

Fig. 3 illustrates in similar diagrammatic showing an embodiment of the invention in which the reproducing lens system is composed of a principal lens having a small spheric error and an additional lens having substantial spheric errors. The electron beam after having passed through the diaphragm 5, in similar manner as shown in Fig. 1,. passes first through a magnetic lens 85 with small spheric aberration and then through an electric lens I l having a considerable larger spheric error. A magnetic lens having a small spheric error is obtained, as is well known, when the coil diameter is made relatively large and the length of the coil is made relatively short. The electric current for energizing the magnetic lens is supplied in known manner by a battery (not shown). The voltage for the electric lens M, illustrated by way of example as composed of two axially alined annular electrodes, are taken from the potentiometer I3. By Varying the tap from the potentiometer the spheric error of the lens may be varied as desired within certain limits.

What we claim as our invention is:'

1. The method of reducing the chromatic error of electron lenses in Braun tubes, in which the cathode ray beam composed of electrons having differential speeds passes through a spherically 5 non-corrected electron lens having a relatively small aperture with respect to the diameter of the cathode'ray beam at the point where the lat ter passes through the electron lens, including the step of adjusting the potential applied to said 1 electron lens to such a value that the spherical error of the lens becomes substantial to refract the electrodes emitted at higher speed and which tend to move away from the optical axis of the cathode ray beam in such manner that their 15 point of intersection almost coincides with the point of intersection of the other electrons emitted at lower initial speed and which travel closer along the optical axis of said cathode ray beam thereby reducing the chromatic aberration normally caused by the slower speed electrons.

2. The method of reducing the chromatic error of electron lenses in Braun tubes, in which the cathode ray beamcomposed of electrons having differential speedspasses through a spherically g non-corrected electron lens including the step of adjusting the potential applied to said electron lens to such a value that the spherical error of the lens becomes substantial to refract the electrons emitted, at higher speed and which tend to 30 move away from the optical azn's of the cathode ray. beam in such manner that their point of intersection almost coincides with the point of intersection of the other electrons emitted at lower initial speed and which travel closer along 35 the optical axis of said cathode ray beam thereby reducing the chromatic aberration normally caused bythe slower speed electrons.

PAUL GoRLIoH. MARTIN PLOKE. 

